1. Field of the Invention
The present invention relates to a touch panel device that determines a touch position of an object by detecting attenuation position of a surface acoustic wave due to a touch of the object. The touch panel device is used as an input device of a personal computer or a personal digital assistant, for example.
2. Description of the Prior Art
As an input device of a personal computer, a mobile computer, a personal digital assistant device (PDA) or the like, the touch panel device is often used in which information can be entered with the touch of a finger or a pen on a display screen of a display device.
There are two types of the touch panel devices. One utilizes a resistance film, and another utilizes a surface acoustic wave (SAW). The resistance film type has a multilayered resistance film in a touch area, which scatters light so that transmittance is low. A touch panel device of the surface acoustic wave type has transducers that are arranged at four sides of the touch area for emitting or receiving the surface acoustic wave. When a finger or the like touches the touch area, the touch position is detected in accordance with the attenuation position of surface acoustic wave. The surface acoustic wave type has an advantage of a high transmittance, a good visibility of the display screen and a high durability against a scratch because the touch area has no resistance film or the like.
The applicant proposed a structure of the surface acoustic wave type touch panel device in Japanese unexamined patent publication 2004-171213. This structure has a single phase transducer (SPT) of an electrode structure in which a piezoelectric thin film is sandwiched between a comb-like electrode and a plate electrode so that only one electrode is disposed on one surface. The structure also has a chevron type electrode structure in which dog-legged comb-like electrodes are arranged in a row.
The touch panel device includes a rectangular transparent substrate and total four transducers. Emitting transducers are disposed at the upper end portion and the lower end portion of the substrate while receiving transducers are disposed at the left end portion and the right end portion. The portion surrounded by the four transducers is the touch area. Each of the transducers has the SPT electrode structure described above and the chevron type electrode structure.
Each of the transducers has one end in the longitudinal direction where a wiring electrode and a connection portion between the comb-like electrode and the plate electrode are disposed closely to each other. An excitation voltage supplied via the wiring electrode is applied to the connection portion so that signal power supply is performed. In addition, a received signal is obtained from the connection portion to the wiring electrode so that signal fetch is performed. The other end of each of the wiring electrodes is drawn as a wire connection portion to one position of the substrate and is connected to a signal process circuit via a flexible cable or the like that is attached to the wire connection portion.
The excitation voltage is applied to the transducers disposed at the upper end and the lower end portions so as to generate surface acoustic waves. The generated surface acoustic wave propagates on the substrate in a diagonal direction and is received by the transducer disposed at the right or the left end portion. When a finger, a pen or the like touches a point in the touch area, the surface acoustic wave is attenuated at the touched point. Therefore, the touched position can be detected by a signal process in accordance with the position where a level of the received signal is attenuated.
In the above-mentioned touch panel device, it is desirable to enlarge the touch area TE and reduce an area for transducers as much as possible. Especially, a small type touch panel device that is used for a PDA or the like is required to have a narrow bezel in which transducers and wiring electrodes are embedded for realizing a compact size of the PDA or the like.
The above-mentioned SPT structure of transducer, in which only one electrode is disposed on each surface of a piezoelectric thin film, has an advantage for reducing its width which corresponds to a width of a bezel portion, to an opposed electrode structure in which two electrodes are disposed on one surface. Therefore, the former has an advantage for reducing a width of a bezel to the latter.
However, wiring electrodes for supplying power to transducers are arranged along the outer rims of the transducers, so the portion for the wiring electrodes should be made in a small width as much as possible. However, in order to reduce the width of the wiring electrode, it is necessary to increase a height of the wiring electrode so as to maintain a cross-sectional area thereof for preventing increase of resistance thereof. It is not good to increase a thickness of the touch panel device for increasing a height of the wiring electrode.